(Not Applicable)
The rapid identification of explosives, explosive residues, chemical agents, airborne toxins, and other volatile organic compounds has undergone a revolution in recent years by the progress made in the field of ion mobility instruments. Despite the transformation that has occurred in ion mobility spectrometry, the full potential of the technique has not yet been realized. This is primarily due to the ion detection systems employed in mobility spectrometers.
FIG. 1 shows a typical ion mobility spectrometer (IMS) 5 to include an ionization reaction chamber 10 in which gas 7 enters and is ionized, an ion drift chamber 20 connected in series with reaction chamber 10 through an ion/molecular injection shutter 12, and a collector plate 16. In operation, a carrier gas transports gases or vapor from material to be analyzed into the reaction chamber 10, where it is ionized by an ionization source. Most of the resulting ions are from the carrier gas molecules, and multiple collisions occur between ionized species and the analyte molecules. These collisions transfer ion charge to the analyte molecules.
For improved resolution, an aperture grid 17 serves as a guard for the collector plate 16 to prevent precharging of the collector due to charging by the approaching xe2x80x9cion packetxe2x80x9d. This grid also helps maintain the uniformity of the electric field responsible for motion of the ions. Periodically, the ion shutter 12 (a charged grid) is opened to allow a pulse of ions into the drift chamber 15. The time of arrival of each ion species at the collector plate 16 is determined by the mobility of the ions in a non-ionizing gas filling the drift chamber 15. The quantity of ions collected at collector plate 16 as a function of drift time is recorded as a current by a microprocessor.
The current state-of-the-art detection limit using a Faraday cup is around 6 thousand ions per second in the most expensive isotope ratio mass spectrometers. This corresponds to 1 femtoamp of current. It takes very sophisticated electronics to measure this small current with any certainty. The direct current measuring devices used in prior art ion mobility spectrometers are able to only measure currents in the picoamp range. This raises the detection limit of the device three orders of magnitude to over 6 million ions per second. This limitation requires high ion fluxes that cause poor linear dynamic range, false positive responses, and numerous other problems.
Because the use of electron multipliers at atmospheric pressures is problematic, researchers have attempted direct electrometer measurements on the very small signals produced by ion packets. Two general methods have been employed.
In the first method, electrometer measurements use amplifiers with extremely high input impedance to measure the voltage resulting from the flow of current through high-precision high-value resistors (typically 1010 to 1012xcexa9) or from the accumulation of charge on a small input capacitor. Limitations on the smallest detectable current by such methods arise from noise effects in the input resistor and from the variable capacitance of the ion collector and cables inherent in the device. High-value input resistors are typically used to produce voltages from the small ion currents. For a 1012xcexa9 input resistor, the thermal noise arising from thermally induced charge fluctuations amounts to about xc2x11 femtoamp at room temperature. The current is determined from a voltage change at a given time. The noise in the rate-of-charge measurement arises from voltage fluctuations in the amplifier. These fluctuations cause relatively high detection limits.
The other method to determine the charge is to measure the change in a capacitor. The same noise fluctuations are involved in this method, and produce similar detection limits. Furthermore, detection limits of several thousand ions per second have not yet been realized for portable ion mobility spectrometers using the existing technology.
A whole new generation of promising ion detectors has been developed based on infrared multiplexer arrays used for night vision and in astronomy. Detection of infrared radiation requires the use of materials with lower band-gaps than silicon devices can offer. The infrared-active materials used are, however, not suitable for the readout multiplexer electronics required for ion detection. Infrared focal plane detectors are often constructed by placing infrared active materials on top of silicon readout multiplexers. These xe2x80x9cstacked systemsxe2x80x9d are capable of reading extremely low levels of charge with exceptionally low read-out noise and low dark currents. An entire generation of ion detectors has been realized from combining the technologies developed for CCD""s, infrared multiplexers, and Faraday ion detection. The result is the micro-Faraday element array capable of low charge detection with ultra-low noise characteristics.
U.S. Pat. No. 6,180,942 of D. Tracy et al. discloses at FIG. 4, an array of ion detectors having a plurality of pickup electrodes separated from a substrate by an insulating layer. Each electrode has associated therewith an undefined xe2x80x98charge storage circuit 38xe2x80x99 where stored voltage is measured before and after sampling. After correction for thermal noise, a multiplexing circuit reads the outputs of the charge storage circuits.
U.S. Pat. No. 6,480,278 of S. Fuerstenau et al. discloses a device for measuring the charge on individual charged particles. The device has a plurality of very small collector plates for measuring charges on droplets, each plate being connected to a CMOS device that includes sample and hold capacitors. The patent indicates a charge transimpedance amplifier may be utilized, but does not show such an embodiment. The patent also stores charge on the element, which means the device is adversely affected by nearby capacitance.
U.S. Pat. No. 5,602,511 of J. Woolaway discloses several capacitive transimpedance amplifiers. FIG. 3 of this patent discloses a CTIA that may be used for signals of different amplitudes by controlling the amount of feedback capacitance.
It is an object of this invention to provide a novel ion collection/detection system for an IMS.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, is an IMS comprising a drift tube for the passage of ions toward an ion collecting surface covering a collecting area at one end of the tube, the surface comprising a plurality of closely spaced conductive elements on a non-conductive substrate, each conductive element being electrically insulated from each other element. A plurality of CTIAs are adjacent the collecting surface, each CTIA having a high impedance input and a charge storage capacitor. Each of the elements is electrically connected to an input of one of said CTIAs, so charge from an ion striking the element is transferred to the capacitor of the connected CTIA. Control means are provided for determining the charge on the capacitors over a period of time.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.